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Heavy rain, but not enough for a CSO

Steady rain last week tested Portland’s new combined sewer tunnel system, but the system performed well and there were no combined sewer overflows (CSOs) to the Willamette River.

Before the West Side and East Side big pipes were in service, one-tenth of an inch of rain in 24 hours was enough to cause a CSO. The roughly 3.5-inches of rain that fell on Portland last week would have caused millions of gallons of combined sewage to overflow to the river. But the 14-foot diameter West Side tunnel and the 22-foot diameter East Side tunnel collected and conveyed all of last week’s wet weather flow to the Columbia Boulevard Wastewater Treatment Plant.

The treatment plant increased capacity to accommodate flow from the CSO tunnel system. From March 10 to March 16, the plant treated 1.1-billion gallons of sewage. That matches a record weekly flow in January and is far more than the plant ever treated in one week before the city activated the CSO tunnels.

Portland’s Willamette River combined sewage system is required to capture winter storms of about 1.2 inches of rain in 24 hours. By early Tuesday, March 13, about 1.5 inches of rain had fallen in 24 hours and the tunnel system was about 96% full. Despite the amount of rain, city wastewater operators were able to capture the whole storm.

Before the start of Portland’s CSO control program, there were 55 active CSO outfall pipes on the Willamette River, and there were an average of 50 CSO events a year. Portland completed the 20-year program in December 2011. Now 25 CSO outfalls are active, and there will typically be no more than an average of four Willamette River CSOs per winter and one every three summers.

Portland’s green stormwater management infrastructure also reduces the amount of stormwater runoff that enters the combined sewer system. The city offers incentives to construct new ecoroofs, which are vegetated roof systems that absorb rain to reduce runoff. Portland is also expanding the number of green streets, which are small rain gardens that collect and filter stormwater runoff from streets. In neighborhoods with combined sewer systems, green stormwater facilities manage an estimated 3-billion gallons of stormwater annually.